A process of fundamental importance in immunology in binding of multivalent antigen to cell-surface antibody which leads to transmembrane signaling in specialized cells. We intend to analyze molecular details of that process by examining a relatively well defined system: the interaction of antigen an immunoglobulin E (IgE) with mast cells and basophils in the allergic response. In our proposed experimental system, monoclonal IgE with known hapten specificity is bound very tightly to the surface receptors of rat basophilic leukemia (RBL) cells, a tumor cell line.. The binding of simple haptens and more complex antigens to this cell-bound IgE will be quantified. Parameters which control the kinetics and thermodynamics of binding will be investigated in order to determine the nature and extent of multivalent interactions between antibody and antigen in this system. In particular, we will be able to control the density of the hapten-specific IgE molecules on the cell surface, and this will allow us to examine the role of lateral mobility of these antibodies in the cell membrane. The dependence of multivalent binding on structure of the antigen also will be investigated by comparing bivalent haptens with different lengths between haptenic groups and with different amounts of structural rigidity. The binding properties of these bivalent haptens will be correlated with the ability to trigger degranulation of the RBL cells, and this may provide some insight to the mechanism of the transmembrane signal in the allergic response. Useful generalizations regarding the roles of cell surface immunoglobulin involved in other immunological responses may also be obtained. It is likely that the segmental flexibility of an antibody molecule affects its interaction with multivalent antigen, and, as part of evaluating the universaity of our experimental system, we will measure the segmental flexibility of the IgE molecule for comparison with other classes of immunoglobulin.